Seed phrase
The 12 or 24 words that unlock a crypto wallet. Whoever has them owns the money.
In plain words
Your seed phrase is the master key to your wallet. It can rebuild the wallet on any device. No real company, exchange or support agent will ever ask for it.
Why it matters if you've been scammed
If someone saw your seed phrase, treat the wallet as lost. Move anything left to a new wallet with a new seed phrase, then follow what to do first.